1. Field of the Invention
The present invention relates to a voltage-to-current converter circuit, and in particular, relates to a voltage-to-current converter circuit suitable for high precision two-wire current signal transmission.
2. Description of the Prior Art
Generally, a voltage-to-current converter circuit is used in the ouput section of a two-wire analog signal transmission system of industrial instruments or the like. FIG. 1 is a characteristic chart showing the characteristics of output current I versus input voltage V of such a voltage-to-current converter circuit. Such a voltage-to-current converter circuit, as shown by a characteristic curve 100, is designed to convert a voltage signal between input voltages V.sub.1 and V.sub.2 to an output current between I.sub.1 and I.sub.2, and for example, a certain input voltage V.sub.1 is accurately converted to an output current I.sub.1, for example of 4 mA, and a voltage V.sub.2 is also converted to a current I.sub.2, for example of 20 mA. Further, a characteristic curve 200 shows a current which is required to operate the voltage-to-current converter circuit, and this current flows in the converter circuit besides the output current I. It is necessary for the voltage-to-current converter circuit, to make this operation current necessary for circuit operation as small as possible and to make the operation current scarcely affect the input voltage versus output current characteristics as shown by the curve 100 even when the operation current varies.
FIG. 2 is a circuit diagram of a voltage-to-current converter circuit known in the art. In the voltage-to-current converter circuit shown in FIG. 2, an input voltage V.sub.i applied to a terminal 2 is level shifted by a level shift circuit 6 connected between the terminal 2 an the negative pole of a power supply 4, and then inputted to an amplifying circuit 8. The amplifying circuit 8 is arranged to supply to a load 10 with a current corresponding to a difference between an input signal voltage V.sub.iL and a feedback signal voltage V.sub.R. The current flowing the load 10 is detected by a feedback circuit 14 including a reference resistor 12, and fed back to the amplifying circuit 8 as a feedback signal voltage V.sub.R. Thus, the voltage-to-current converter circuit supplies through the load 10 with a current proportional to the input voltage V.sub.i. The level shift circuit 6 includes a resistor 61 of a resistance value R.sub.1 and a resistor 62 of a resistance value R.sub.2 connected in series, and the input voltage V.sub.i voltage divided by the resistors 61 and 62 is supplied to the amplifying circuit 8. The amplifying circuit 8 is comprised of an operational amplifier 81 and a transistor 82, in which the operational amplifier 81 introduces a difference between the input signal voltage V.sub.iL and the feedback signal voltage V.sub.R and amplifies the difference as a differential amplifier. The transistor 82 on the other hand, controls a current from the power supply 4 in accordance with the output signal from the operational amplifier 81. The positive pole of the power supply 4 is connected to the collector of the transistor 82 and to the power supply terminal of the operational amplifier 81 via a load 10. Furthermore, the emitter of the transistor 82 is connected to the negative pole of the power supply 4 via the reference resistor 12, and the emitter is also connected to ground. The negative power supply terminal of the operational amplifier 81 is connected to the negative pole of the power supply 4.
The voltage-to-current converter circuit arranged as described above operates in the following manner. The current output from the current output transistor 82 is detected as a voltage drop across the reference resistor 12 provided on the emitter side of the transistor 82. The voltage across the resistor 12 is inputted to the non-inverting input of the amplifier 81 via the resistor 62 of the level shift circuit 6. The input voltage V.sub.i applied to the terminal 2 is also inputted to the non-inverting input of the transistor 81 via the resistor 61. The amplifier circuit 81 controls the flow of current through the load 10 so that a current corresponding to a difference between the input signal voltage V.sub.i and the feedback signal voltage (substantially same with the voltage drop of the reference resistor 12) V.sub.R flows. In other words, in the operational amplifier 81, the difference between the input signal voltage V.sub.i and the feedback signal voltage V.sub.R is detected and the difference is amplified thereby to supply the amplified output to the base of the transistor 82. In accordance with the control signal applied to the base of the transistor 82, a current I.sub.0 corresponding to the input signal voltage V.sub.i flows through the load 10 and the reference resistor 12.
The output current characteristics of the voltage-to-current converter circuit described above is expressed by: ##EQU1## where R.sub.0 is the resistance value of the reference resistor 12, R.sub.1 and R.sub.2 are respectively resistance values of the resistors 61 and 62, I.sub.0 is the output load current, V.sub.1 is the input signal voltage, and I.sub.CC is the operation current (which flows in and out through the power supply terminals of the operational amplifier) of the operational amplifier.
As will be apparent from the equation (1), in the voltage-to-current converter circuit of FIG. 2, since the operation current I.sub.CC of the operational amplifier 81 does not flow the reference resistor 12, the I.sub.CC component is not detected by the feedback circuit 14 and hence it causes an error in the input voltage versus output current characteristics. In addition, the value of the operation current I.sub.CC is usually, of the order of mA, and it is not negligible as compared with the value of the output current (maximum several tens mA). Furthermore, in the prior art converter circuit, since precision resistors must be used for the resistors R.sub.1, R.sub.2 and reference resistor R.sub.0, there is the drawback that many expensive precision resistors are required.
In order to eliminate such a drawback, a voltage-to-current converter circuit as shown in FIG. 3 was devised. In FIG. 3, like reference characters are used for like constituent elements as in FIG. 1, and descriptions thereof are omitted. The basic difference in the arrangement of FIG. 3 with respect to that of FIG. 2 resides in that the entire current flowing from the power supply 4 through the amplifying circuit 8 is made to pass through the reference resistor 12. For this purpose the position of the reference resistor 12 differs from that in FIG. 2. In order to obtain still a sufficient operation margin of the operational amplifier 81 irrespective of the change in the position of the reference resistor 12, an output compensating circuit 16 is connected between the emitter of the output transistor 82 of the amplifying circuit 8 and the reference resistor 12, and further an input compensating circuit 18 is provided on the input side of the amplifying circuit 8. The output compensating circuit 16 includes a zener diode ZD.sub.1 connected between the reference resistor 12 and the emitter of the output transistor 82. Furthermore, the circuit arrangement of FIG. 3 is so designed that the entire current including a current flowing the output transistor 82, a power supply current to the error amplifier (operational amplifier) and a current flowing through a series circuit of a resistor 183 and a zener diode ZD.sub.2, that is, the load current I.sub.0 flows the reference resistor 12. Such a circuit arrangement has been publicly known, for example, from U.S. Pat. No. 3,654,545 issued Apr. 4, 1972 to Anthony M. Demark, entitled "Semiconductor Strain Gauge Amplifier", in which a basically similar circuit arrangement is disclosed.
In addition, the input compensating circuit 18 voltage divides the feedback signal voltage V.sub.R from the feedback circuit 14 by resistors 181 and 182 to apply the divided voltage to the non-inverting input terminal of the operational amplifier 81 of the amplifying circuit 8, and also voltage divides the input voltage V.sub.i by the level shift circuit 6 to apply the divided voltage to the inverting input terminal of the operational amplifier 81. Furthermore, the input terminal of the resistor 182 for the input signal V.sub.i is connected to the cathode of the zener diode ZD.sub.2 and this junction point is connected to the collector of the transistor 82 via a resistor 183, and the anode of the zener diode ZD.sub.2, one of the power supply terminals of the operational amplifier 81 and the anode of the zener diode ZD.sub.1 whose cathode connected to the emitter of the output transistor 82 are connected in common to ground.
The input output characteristics of such a voltage-to-current converter circuit is expressed by the following equation: ##EQU2## where k.sub.1 is the ratio ##EQU3## k.sub.2 is the ratio ##EQU4## and V.sub.Z is the voltage of the zener diode ZD.sub.2. According to this voltage-to-current converter circuit, since the entire current flowing through the load 10 also flows through the resistor 12, the operation current I.sub.cc of the amplifier 81 does not appear as an error. However, since the precision of the output current are dependent on the precision of k.sub.1 or k.sub.2 of the voltage dividing resistors, it was difficult to improve the precision and to reduce the costs.